The crayfish eyestalk (ES) has been postulated as a possible circadian clock. 5-Hydroxytryptamine (5-HT) has been shown to play the role of a neurotransmitter or a modulator in the ES. However, little is known about the 5-HT receptors in the ES. The purpose of this work is to determine the specific binding sites using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin ([(3)H]8-OH-DPAT), a specific agonist of the 5-HT(1A) receptor, and to characterize the diurnal rhythm in the binding by an autoradiography procedure in the crayfish ES. Data show the presence of a circadian rhythmicity in the level of the 5-HT(1A) receptors, principally in two regions: (a) the complex retina (R)-lamina ganglionaris (LG), with the acrophase at dusk and (b) the medulla terminalis (MT), where it was in antiphase. It is suggested that (1) the expression of levels of 5-HT(1A) receptors is modulated by light-dark (LD) cycles, (2) the level of 5-HT(1A) receptors in the R-LG and MT are in antiphase during the 24-h cycle, and (3) there is a different mechanism of action of LD cycles in each of these two anatomical regions of the crayfish ES.